Synaptic connections in neural pathways involving giant interneurons are being investigated by anatomical and electrophysiological methods in crayfish and earthworm. Specifically, the number, soma position, and soma size of deep flexor motoneurons have been determined in the anterior 5 of the 6 abdominal ganglia in crayfish. Synaptic connections of these neurons to the medial and lateral giant interneurons and to muscles are under study. In earthworms afferent pathways to the lateral giant interneurons are being determined. In both of these studies the primary interest is evolutionary and developmental: How have patterns of synaptic connections of serially homologous neurons become specialized in relation to the distinctive roles each segment plays in behavior? What ontogenetic mechanisms produce differential specialization of serially homologous neurons? Regeneration studies to approach the latter question are underway. BIBLIOGRAPHIC REFERENCES: Mittenthal, J.E. 1975. A sliding filament model for skeletal muscle: Dependence of isometric dynamics on temperature and sarcomere length. J Theor. Biol. 52: 1-16. Mittenthal, J. E. and Wine, J.J. 1975. Segmental homology and variation in flexor motoneurons of the crayfish abdomen. Neuroscience Abstr. 1: 590.